1. Field of the Invention
The present invention relates to a method for manufacturing molten metal containing Ni and Cr by directly smelting and reducing Ni ore and Cr ore in a smelting reduction furnace by the use of carbonaceous material as fuel or reducing material.
2. Description of the Prior Art
Materials such as scrap, ferro alloy, electrolytic Ni or the like have previously been used in melting and refining in a process of manufacturing stainless steel. Those materials are melted in an electric furnace or a converter. Cr and Ni being major components of stainless steel in the state of ferrochrome and ferronickel are charged into the electric furnace or the converter. Chromium ore and nickel ore are required to be previously reduced in the electric furnace or the like to manufacture ferrochrome and ferronickel. Expensive electric energy is used for reducing chromium ore and nickel ore. Accordingly, a method wherein ferrochrome and ferronickel are used as materials is not economical method.
On the other hand, four methods, which will be described as follows, are pointed out as methods of using inexpensive material for a source of Ni. A first method is a method wherein molten metal of FeNi is directly used in an electric furnace for the purpose of reducing a cost of melting FeNi. This method is described in the magazine "Iron and Steel" [69 (1983) 7, p. 59]. A second method is a method for smelting and reducing nickel matte in a converter which is disclosed in a Japanese Patent Publication Laid Open No. 104153/83. A third method is disclosed in a Japanese Patent Publication Laid Open No. 36613/85. In this method, material produced by mixing nickel oxide with carbonaceous material and forming a mixture thus obtained is heated and prereduced and the material prereduced is charged into a converter type reaction vessel, in which it is smelted and reduced. A fourth method is a method wherein nickel oxide is used (a Japanese Patent Publication Laid Open No. 291911/86).
Moreover, several methods wherein Cr ore is used as a source of Cr and Cr ore is smelted and reduced in a converter or other melting furnaces have been proposed. There are known, for example, a method wherein Cr ore is smelted and reduced by blowing oxygen from a top-blow oxygen lance and, at the same time, blowing oxygen from bottom-blow tuyeres and blowing nitrogen from side-blow tuyeres and a method wherein Cr ore is smelted and reduced by blowing oxygen from bottom-blow tuyeres and oxygen and nitrogen from side-blow tuyeres respectively. As an example of the latter, Japanese Patent Publication Laid Open No. 279608/86 can be pointed out.
Next, problems of the prior art will be studied. In the prior art method for manufacturing molten metal containing Ni, Ni ore is not smelted and reduced by directly charging Ni ore into the smelting furnace. Since Ni ore has a low content of 2 to 3 wt. % Ni and about 80% of weight of Ni ore converts to slag, Ni ore produces a great amount of slag during smelting reduction of the Ni ore. Accordingly, when it is tried to obtain a predetermined concentration of Ni, a great amount of slag is produced. For example, when molten metal containing 8 wt. % Ni is obtained, 2 to 3 t of slag per ton of molten metal are produced. In connection with the production of a great amount of slag, the following problems become remarkable.
(1) Slopping is liable to be produced by a reaction of oxygen with carbonaceous material charged into the smelting reduction furnace as a reducing material or as a heat source at the step of the smelting reduction. In consequence, a stable operation becomes difficult whereby the operation becomes unstable.
(2) Equipment is greatly damaged by the slopping; and
(3) The yield of Ni is decreased by the slopping.
In view of the foregoing problems, in the foregoing citations, Ni ore as a source of Ni is not directly charged into the smelting reduction furnace, but material having a content of Ni, whose percentage in the material is increased by subjecting the material to some preliminary treatment, is used.
On the other hand, since chromium oxide as a source of Cr is difficult to melt and considerable energy is required to reduce chromium oxide, a rate of reduction of chromium oxide in the prior art smelting reduction methods is small and requires much time to process chromium oxide. Such problems are posed by the following reasons:
1 A reduction of Cr ore in the smelting reduction furnace progresses under the reaction of C in the carbonaceous material with melted Cr ore after the Cr ore has been melted in slag. The rate of reduction of Cr ore is strongly affected by the rate of melting of Cr ore. Therefore, major technical attention for shortening a processing time has been paid to a determination of slag components or the like. However, Cr ore is fundamentally hard to melt and there is a limit of increasing the rate of reduction of the Cr ore by promoting the melting of the Cr ore.
2 A method, wherein CO gas in the melting furnace is post-combusted and post-combustion heat is used to increase a melting rate of Cr ore in slag and to increase a reaction rate of the Cr ore, is conceivable. A method of blowing post-combustion oxygen from an upper portion of walls of the melting furnace is employed in the prior art methods as well. In the prior art, however, although a temperature of exhaust gas rises when a post-combustion ratio is raised, there is no technology for effectively transferring sensible heat of the exhaust gas to molten metal. As the result, the heat transfer efficiency decreases and high-temperature exhaust gas is exhausted. There occurs a problem such that the high-temperature exhaust gas cause rapid wearing of the refractory of an exhaust gas hood. Therefore, it has been generally thought that the post-combustion ratio cannot be raised too high.
Molten metal containing Ni and Cr can be obtained by smelting and reducing Ni ore and Cr ore. The molten metal containing Ni and Cr is subjected to a decarbonization treatment so as to obtain stainless steel. The decarbonization treatment is desired to be continuously carried out in the same furnace as that in which the smelting reduction is carried out. The method wherein decarbonization treatment of molten metal carried out in the same furnace after the smelting reduction of Ni ore and Cr ore has previously not been substantially studied. This is explained by the following reasons:
(1) When the molten metal is subjected to the decarbonization treatment in a converter type vessel, there occurs a problem such that an oxidation loss of Cr is remarkable. Therefore, even though the smelting reduction of Ni ore and Cr ore is practically carried out in the converter type vessel, a vacuum method such as a RH-OB method, in which the oxidation loss of Cr is small, has to be adopted for the decarbonization treatment.
(2) A great amount of stirring gas is required in the decarbonization treatment. In the prior art smelting reduction, a great amount of stirring gas necessary to carry out the the decarbonization treatment is not thought to be supplied. Therefore, even though the same converter type vessels are used in the smelting reduction and decarbonization treatment, both of the furnaces have been thought to have different structures.
(3) A great amount of slag produced by the smelting reduction of Ni.sub.1 ore and Cr ore is required to be discharged out of the furnace to carry out processes from the smelting reduction to the decarbonization in the same furnace. However, electric furnaces usually used do not have a structure capable of discharging slag.
(4) The time for the smelting reduction and decarbonization is long in the prior art smelting reduction method and decarbonization method. Accordingly, when the smelting reduction and decarbonization are carried out in the same furnace, it takes very much time for carrying out all the treatments. In consequence, industrial operations are hard to carry out due to a decrease of productivity and a great wear of the refractory of the smelting reduction furnace.